1. Field of the Invention
The present invention relates to a method and apparatus for header setting in a wireless communications system, and more particularly to a method and apparatus for decreasing header length of radio link control protocol data units.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates.
The access stratum of the 3G mobile telecommunications system comprises a radio resource control (RRC), radio link control (RLC), media access control (MAC), packet data convergence protocol (PDCP), broadcast/multicast control (BMC) and other sub-layers of different functions. The operations of the above-mentioned sub-layers are well known for those skilled in the art, and will not be further mentioned. A primary function of the RLC layer is providing different transmission quality processing, performing segmentation, reassembly, concatenation, padding, retransmission, sequence check, and duplication detection on transmitted data or control instructions based on different transmission quality requirements. The MAC layer can match packets received from different logic channels of the RLC layer to common, shared, or dedicated transport channels according to radio resource allocation commands of the RRC layer, for performing channel mapping, multiplexing, transport format selection, or random access control.
In the RLC layer, the purpose of “padding” is to make lengths of all Protocol Data Units (PDUs) outputted from an RLC entity to be the same. That is, after the RLC entity receives a Service Data Unit (SDU) from the upper layer, if the length of the SDU is smaller than a maximum PDU payload size, the RLC entity will pad out the SDU with meaningless data, so as to form a PDU in conformation with a predefined length. Otherwise, if the length of the SDU is greater than the maximum PDU payload size, the RLC entity will segment the SDU with the maximum PDU payload size. After the segmentation, if the length of the last segment is smaller than the maximum PDU payload size, the RLC entity will pad out the last segment with meaningless data, to form a PDU in conformation with the predefined length.
In the prior art, “padding” can make the lengths of the RLC PDUs to be identical to each other. However, the prior art “padding” decreases bandwidth utility rate and data processing efficiency, especially for high data rate applications, such as High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA) in the 3G mobile telecommunications system. In such a situation, a scheme of variable RLC PDU size is proposed to eliminate padding, to enhance bandwidth utility rate and data processing efficiency, and to improve uplink and downlink (UL/DL) transmission rate.
For example, please refer to FIG. 1, which illustrates a schematic diagram of an application of the variable RLC PDU size according to the prior art. In FIG. 1, SDU_1 and SDU_2 represent SDUs from the upper layer, PDU_1˜PDU_3 represent RLC PDUs, oblique-line parts in front of PDU_1˜PDU_3 represent PDU headers, and MPZ represents the maximum PDU payload size. As shown in FIG. 1, the total length of SDU_1 and SDU_2 is greater than two times MPZ, but smaller than three times MPZ. Therefore, the prior art uses a “Blind Cut” method to carry SDU_1 and SDU_2 with PDU_1˜PDU_3. The Blind Cut method means that if the length of one or concatenated SDUs is greater than one or multiple times of MPZ, the RLC entity will segment the SDU with a unit of MPZ until a last segment or SDU smaller than MPZ is left, and carry the last segment or SDU with a variable-size RLC PDU. In other words, the lengths of PDU_1 and PDU_2 are equal to the maximum PDU size, such as 42 bytes, while the length of PDU_3 is smaller than the maximum PDU size, such as 29 bytes.
In the prior art, a Length Indicator (LI) field, 7-bit or 15-bit, is needed in the RLC PDU to indicate an ending position of an SDU. Following the LI field, there is an Extension bit (E) field indicating that the next field is LI or data field. Under the condition that “padding” is unnecessary, the LI field may become redundant, thus increase the length of the PDU header, and affect transmission efficiency.